This invention relates to a matrix circuit for adding and subtracting a plurality of signals suitable for embodying a color matrix circuit of PAL/SECAM compatible type color TV receiver.
As is generally known there are three standard color TV systems, i.e. NTSC,
and SECAM systems. Any nation practicing a color TV broadcasing uses one of these three standard systems. In these systems PAL and SECAM are used mainly in European and African nations. Thus, PAL system and SECAM system are employed in neighbouring countries. These different color broadsting systems possess common parts except the part for processing a chrominance signal. Accordingly, the demand for a special TV receiver which is compatible with the PAL and SECAM systems increases as color TV receivers are gradually being popularized in European and African nations.
The chrominance signal of color TV signals used for PAL system is composed by the following steps. Tricolor signals R, G and B obtained from a color TV camera are converted by a color matrix circuit into a luminance signal Y and color difference signals (R-Y) and (B-Y). The color difference signals are subjected to a frequency band restriction. Then the band-restricted color difference signals are used in a two-phase quadrature amplitude modulator, for modulating a chrominance subcarrier to provide the carrier chrominance signal. In the modulator the phase of (R-Y) modulation axis is phase-inverted at intervals of each horizontal scanning period. The advantage of PAL system that the demodulated chrominance signal is little affected by a phase distortion in the subcarrier is derived from the following processings. First, a signal with a delay corresponding to one horizontal scanning period (hereinafter referred to as "1H"), i.e. a delayed signal travelling a 1H delay line, and a nondelayed signal bypassing the 1H delayed line are obtained from the carrier chrominance signal. Second, the delayed and nondelayed signals are combined in a PAL matrix circuit to produce (B-Y) and (R-Y) carrier chrominance signals. The above-mentioned advantage will be obtained from averaging the signals of two horizontal scanning periods (2H).
In the SECAM system a chrominance signal of color TV signals also provides color difference signals (B-Y) and (R-Y). During the transmission of the (R-Y) carrier chrominance signal through a 1H delay line, the (B-Y) carrier chrominance signal is outputted. Then the alternative transmitted carrier chrominance signals are obtained simulataneously.
In a prior art TV receiver being compatible with PAL and SECAM systems, each of PAL and SECAM circuits equips an exclusive 1H delay line for the individual system. As the circuit integration of color circuitry is scaled up with the progress of semiconductor IC technology, the cost of a 1H delay line relatively increases. If one 1H delay line may commonly be used for PAL and SECAM systems, the cost of a PAL/SECAM compatible type TV receiver will effectively be reduced.
The drawback of the prior art will be discussed with referring to FIGS. 1 and 2. FIG. 1 shows a circuit arrangement of PAL matrix circuit. In this figure one input terminal 1A is grounded through an impedance matching circuit of a resistor 2 and an inductor 3. The other input terminal 1B is grounded. The both terminal ends of inductor 3 are coupled with the input terminals of a 1H delay line 4. Connected to the output terminals of 1H delay line 4 are an impedance matching inductor 5 and input terminals 6A and 6B of a resistance matrix circuit 6. Another input terminal 6C of resistance matrix circuit 6 is connected via a trimmer resistor 7 to the input terminal 1A. Thus, an input signal at the input terminal 1A is transferred to the input terminal 6C without signal delay. The matrix circuit 6 includes a resistor 8 connected between the input terminal 6C and the circuit ground, and a resistor 9 connected in parallel with the resistor 8. The center tap of resistor 8 is connected to the input terminal 6A and also to an output terminal 6D of matrix circuit 6. Similarly, the center tap of resistor 9 is connected to the input terminal 6B and an output terminal 6E of matrix circuit 6.
The circuit configuration of FIG. 1 will operate as follows.
A PAL carrier chrominance signal F(P)n being ahead of a given time which corresponds to 1H has been applied to the input terminal 1A. The signal F(P)n is converted through the 1H delay line 4 to a first carrier chrominance signal F(P)'n with a time delay of 1H. Then the first carrier chrominance signal F(P)'n and a second carrier chrominance signal F(P)n+1 applied to the input terminal 1A are simultaneously inputted to the matrix circuit 6. In the matrix circuit 6, the first carrier chrominance signal F(P)'n and the second carrier chrominance signal F(P)n+1 flow into the resistor 8 of adding side to provide an addition output F(P)A, wherein the signal F(P)'n has an inphase relation with the signal F(P)n+1. Further, the signal F(P)n+1 and the signal -F(P)'n whose phase has an antiphase relation with the signal F(P)n+1 flow into the resistor 9 of subtracting side to provide a subtraction output F(P)S.
FIG. 2 shows a circuit arrangement of SECAM type color signal switching circuit. In FIGS. 1 and 2, for avoiding a redundant explanation, the common circuit component is denoted by the same reference numeral. In FIG. 2, one output terminal of a 1H delay line 4 is connected to an input terminal 10A of a SECAM switching circuit 10, while the other output terminal of 1H delay line 4 is grounded. An input terminal 1A is connected via a trimmer resistor 7 to an input terminal 10B of the switching circuit 10. Carrier chrominance signals are obtained from output terminals 10C and 10D of the switching circuit 10. The switching circuit 10 is formed of a two-ganged type electronic switches 11 and 12. The input terminal 10A is connected to fixed contacts 11a and 12b of switches 11 and 12. The input terminal 10B is connected to fixed contacts 11b and 12a thereof. Movable contact pieces 11c and 12c are connected to the output terminals 10C and 10D, respectively.
In such circuit configuration, when a SECAM carrier chrominance signal F(S)n being ahead of 1H has been applied to the input terminal 1A, the 1H delay line 4 provides a first chrominance signal F(S)'n to the input terminal 10A of switching circuit 10. A carrier chrominance signal F(S)n+1 applied now to the input terminal 1A is inputted as a second carrier chrominance signal to the input terminal 10B of switching circuit 10. The switches 11 and 12 are switched at intervals of the horizontal scanning period (1H). Accordingly the (R-Y) signal appears at the terminal 10C and the (B-Y) signal at the terminal 10D.
The circuit configurations of FIGS. 1 and 2 will operate as mentioned above. These figures teach that balanced outputs are required for the 1H delay line 4 of PAL circuit (FIG. 1) whereas an unbalanced output is necessary to the 1H delay line 4 of SECAM circuit (FIG. 2). Therefore, if only one 1H delay line 4 in a TV receiver should be used for both PAL and SECAM circuits, it is impossible to avoid the use of a special switch for the output circuit of delay line 4. Such special switch, however, increases the manufacturing cost and thus cancels the cost down merit obtained from decreasing two delay lines to one delay line.